Desiccant based air conditioning systems have been finding increased usage during recent years. These systems have been used to solve certain HVAC problems that conventional vapor compression refrigeration systems are ill-equipped to handle. For example, desiccant based air conditioning systems have been used in applications where better humidity control is required. This is due to the fact that desiccant systems are capable of drying the air to a lower relative humidity than conventional systems without frost development.
In addition, desiccant systems have been used where microbiological growth is a concern. Desiccant systems do not require the "wet surface" evaporator coil which is common to conventional systems. This coil, along with its associated condensate collection basin, can create a prime biological breeding ground. Also, tests have shown that some desiccant systems can effectively remove bacteria from the air stream with which the desiccant is brought in contact.
Desiccants can be solid, liquid, or gaseous substances which have as a basic characteristic the ability to attract and hold relatively large quantities of water. If, in attracting and holding moisture the desiccant undergoes a chemical change, the process is called absorption. If, in attracting and holding moisture the desiccant undergoes a physical change only, the process is called adsorption. In general, most absorbents are in liquid form and most adsorbents are in solid form.
In many commercial air-conditioning applications where desiccants are used, the desiccant is in solid form and adsorbs moisture from the air to be conditioned. Examples of these types of desiccants are silica gel, activated alumina, molecular sieves, or hygroscopic salts. In some cases, these desiccants are contained in "beds" over which the air to be conditioned is passed. Many times, however, the desiccant is contained in what is known as a "Desiccant Wheel".
A desiccant wheel is an apparatus typically comprising a plurality of closely spaced, very thin sheets of plastic or metal which are coated with a desiccant material. The wheel is contained in a duct system that is divided into two sections. The wheel is rotated slowly on its axis such that a given portion of the wheel is sequentially exposed to the two sections. In the first section, the desiccant is contacted by the process air, or the air to be cooled and dehumidified. In this section, the desiccant dehumidifies the process air by adsorbing moisture from this air.
In the second section of the desiccant wheel, the desiccant is contacted with the regeneration air. The regeneration air evaporates the moisture from the desiccant that the desiccant adsorbed from the process air, thereby regenerating the desiccant. By the wheel rotating through these two air streams, the adsorbing/desorbing operation of the wheel is continuous and occurs simultaneously.
Generally, the typical system, as shown in Prior Art FIG. 4, operates by passing the air to be conditioned, or process air, through the dehumidification section of the desiccant wheel wherein the air is dehumidified and warmed. This warming occurs from the latent heat of the water adsorbed onto the desiccant and from the heat of adsorption generated by this process. Upon exiting the desiccant wheel, the process air passes through one side of an air-to-air heat exchanger. In this heat exchanger, the process air gives up some of the heat it picked up in the desiccant wheel to the air stream which is to be used to regenerate the desiccant wheel. After passing through the air-to-air heat exchanger, the process air is cooled by passing it through the dry side of an indirect evaporative cooler and then is humidified and further cooled by passing it through a direct evaporative cooler. The cool, moist air exiting the direct evaporative cooler is then supplied to the space to be conditioned. Part of the air leaving the space to be cooled is exhausted and makes up a portion of the regenerative air stream. The remaining exhaust air is recirculated and mixed with ambient air to make up the process air.
The desiccant used to dehumidify the process air must be periodically regenerated in order for it to remain effective at drying the process air. This regeneration is accomplished by passing warm or hot air through the wheel in order to evaporate the water from the desiccant into the air stream. In the typical system, this warm or hot air is made up of ambient air which is first passed through the air-to-air heat exchanger where it picks up some of the heat from the process air. The regenerative air stream is then passed through a heating apparatus to further heat the air before it enters the desiccant wheel. After heating, the regeneration air stream is passed through the regenerative section of the desiccant wheel in which it evaporates moisture from the wheel. The regenerative air stream is exhausted after it passes through the desiccant wheel.
Two general problems are associated with the typical desiccant based air conditioning systems. First, the air-to-air heat exchanger, which is used to transfer the heat energy from the dried process air leaving the desiccant wheel to the regeneration air stream, is costly. This drives up the first cost of the desiccant based air conditioning systems thereby limiting their application. In addition, the amount of heat recovered from the process air and transferred to the regeneration air stream typically only accounts for 30-35% of the total heat energy required for this regeneration air stream. Accordingly, the second problem associated with the typical desiccant system is that these systems require a significant amount of energy to sufficiently heat the regeneration air stream to allow it to effectively dry the desiccant. In some applications where there is a local supply of inexpensive fuel or if there is a supply of waste heat, this is not a problem. However, in the vast majority of applications this will be a significant disadvantage to the use of desiccant based air conditioning systems. A system which required less energy to regenerate the desiccant wheel would reduce the operating cost of desiccant based air conditioning systems thereby making them cost effective in a greater number of applications.